A variety of motors employed in, e.g. air conditioners, often use a permanent-magnet brush-less DC motor (hereinafter referred to simply as a motor), of which rotor is equipped with permanent magnets, taking advantage of its long service-life, high reliability, and easy speed control.
A conventional motor driver for driving such a motor as discussed above is disclosed in Japanese Patent Application Non-Examined Publication No. 2002-369576. This motor driver has a structure such that if the rotor is driven in a reverse direction by external force or inertia when the driver is about to start driving the motor, the driver is prohibited from driving the motor. This structure prevents a high current from running through the motor driving coil wound on the stator teeth, so that no demagnetiziation occurs in the rotor magnets.
To be more specific, a control unit disposed in the motor driver detects a rotary direction of the rotor based on changes of a position detecting signal supplied from a Hall sensor prepared in the motor. When the driver is about to start driving the motor, the control unit permits driving the motor and outputs a control signal for turning on transistors in the inverter section provided that the rotor is halted or the rotor rotates in the same direction as the direction the motor is about to rotate along. The output of the control signal entails a driving current to run through the motor driving coils, so that the motor starts rotating.
On the other hand, when the driver is about to start driving the motor, the control unit prohibits the motor from rotating and outputs a control signal for turning off every transistor in the inverter section provided that the rotor rotates in the reverse direction to the direction the motor is about to rotate along. The output of the control signal results in supplying no driving current to the motor driving coils. This structure prevents a high current from running through the motor driving coils, so that no demagnetiziation occurs in the rotor magnets.
FIG. 6 shows a circuit diagram of another conventional motor driver, and FIG. 7 shows signal waveforms, of which parts illustrate signals in normal energizing and the remaining part illustrates the signals in non-normal energizing of the motor driver shown in FIG. 6.
As shown in FIG. 6, in the motor driver normally energized, energizing signal generator 190 outputs energizing signals UH0, UL0, VH0, VL0, WH0, and WL0. Those signals control six transistors, 131, 132, 133, 134, 135 and 136 to be turned on or off sequentially. Those six transistors form energizing unit 120. This control entails the current supply to the three-phase driving coils 111, 113 and 115 to be switched sequentially as signals U, V and W shown in FIG. 7, thereby rotating the motor. The driving coils are equipped to the stator of the motor.
The foregoing motor driver stops the current supply to driving coils 111, 113 and 115 when the current supplied to energizing unit 120 increases up to a given value. This action is referred to as non-normal energizing. To be more specific, when the current increases, over current detector 170 outputs signal OC, which is received by signal selector 150. Selector 150 then switches signals supplied from energizing signal generator 190 into signals UH1, UL1, VH1, VL1, WH1, and WL1 supplied from energizing signal output unit 140 before outputting them to energizing unit 120. Those signals supplied from output unit 140 turn off all the transistors 131–136 in energizing unit 120. This mechanism stops the current supply to driving coils 111, 113 and 115.
There are methods other than the foregoing method for stopping the current supply to the driving coils, e.g. supply of a signal to turn on transistors 131, 133, 135 and turn off transistors 132, 134, 136, or the other way around, i.e. to turn on transistors 132, 134, 136 and turn off transistors 131, 133, 135.
However, assume that the latter instance is taken as an example of conventional motor drivers, when all transistors 131–136 in energizing unit 120 are turned off, the energy stored in three-phase driving coils 111, 113 and 115 travels through any one of flywheel diodes 121–126, i.e. runs as a current. This run of current sharply changes voltages U, V and W across the driving coils as shown in FIG. 7. As a result, the driving coils vibrate, which sounds audible and sometimes causes noises. Normal energizing and non-normal energizing repeat at a variety of intervals, so that the repeat causes grating noises if it falls within the audio frequency.
As discussed previously, the another method for stopping the current supply to the driving coils 111, 113 and 115 is available: shorting the driving coils each other by turning on transistors 131, 133, 135 and turning off transistors 132, 134, 136, or the other way around, i.e. turning off transistors 131, 133, 135 and turning on transistors 132, 134, 136. The current supply from power supply Vdc can be halted through those methods; however, back electromotive force (BEMF) is generated in the driving coils during the spin and a current caused by the BEMF will flow.
The methods discussed above have the effect of reducing the current running through the driving coils in the case of positive spin (spin-direction driven by the normal energizing). However, in an apparatus of which fan is driven by a motor, when the fan is rotated in a reverse direction (direction opposite to the normal energizing direction) by some external force such as wind energy, the current running through the driving coils sometimes further increases. The increase of current violates the over-current regulating function that should be activated in the normal energizing state. This violation leaves a problem in actual operation.